Electrical resistance varies with temperature and strain. This phenomenon underlies the operation of a bridge transducer used for measuring an effect such as strain or temperature. The transducer contains an electrically activated sensing bridge and a signal-conditioning portion. The bridge is formed with a group of resistors, of which one or more are subjected to the effect being measured. A pair of signals representative of the change in resistance due to the effect are taken from suitable points on the bridge. The signal-conditioning circuit converts the bridge signals into a suitable output form.
A particularly useful type of bridge transducer circuit provides the output signal at a frequency corresponding to the value of the effect under measurement. Friedl et al disclose such a device in "A New Resistance-to-Frequency Converter for Temperature Measurements in Calorimeters", IEEE Transactions on Instrumentation and Measurement, Dec. 1975, pp. 322-324. FIG. 1 illustrates the basic circuitry features of this device.
The six resistors shown in FIG. 1 form a bridge 10 energized by variable voltages V.sub.E1 and V.sub.E2 supplied on lines 11 and 12. Bridge resistors RT1 and RT2 are placed in an environment whose temperature is under investigation. A voltage V.sub.B is taken at the node between the two equal-value resistors RV. With the node between the two equal-value resistors RR connected to a fixed voltage point (ground reference), voltages V.sub.B and V.sub.E1 (or V.sub.E2) are representative of the temperature(s) acting on resistors RT1 and RT2.
An integrator consisting of a capacitor CO connected across a high-grain amplifier 13 integrates a charging current I.sub.C generated from voltages V.sub.B and V.sub.E1 to produce an integrated voltage V.sub.I. A comparator 14 compares voltage V.sub.I with a voltage V.sub.R taken at the node between resistor RT2 and lower resistor RR to produce an output voltage V.sub.O representative of the comparison. Voltage V.sub.O controls the positions of switches 15 and 16 which appropriately connect lines 11 and 12 to supply lines 17 and 18. A floating power supply 19 differentially provides lines 17 and 18 with a bridge supply voltage V.sub.BS.
The operation of this transducer can be understood with the assistance of FIG. 2. The energizing voltage difference V.sub.E1 -V.sub.E2 is termed V.sub.E. If switches 15 and 16 are at the positions indicated in FIG. 1 so as to connect lines 11 and 12 respectively to lines 17 and 18, V.sub.E is approximately V.sub.BS. V.sub.R is at a negative value -U.sub.R. I.sub.C flows into capacitor CO, causing V.sub.I to decrease during a time t.sub.D as shown in FIG. 2.
When V.sub.I falls just below -U.sub.R, V.sub.O changes polarity. This causes switches 15 and 16 to reverse their positions. Lines 11 and 12 are now respectively connected to lines 18 and 17 to reverse the V.sub.E polarity, thereby reversing the polarities of V.sub.R and I.sub.C. V.sub.I rises during a time t.sub.U. When V.sub.I passes U.sub.R, V.sub.O returns to its original polarity to restart the cycle. The V.sub.O switching frequency is proportional to the difference between the values of resistors RT1 and RT2.
The transducer of Friedl et al appears quite accurate. However, implementing the signal-conditioning portion of the device as a semiconductor integrated circuit can become quite complex because power supply 19 is a "floating" supply. Furthermore, the transducer is a four-wire system since a pair of supply lines (not shown) separate from lines 17 and 18 are needed to provide a supply voltage for components 13 and 14.